The invention is related to waste treatment and, particularly, to a method and apparatus for the continuous anaerobic decomposition of organic compounds.
As interest in environmental safety grows, governmental agencies continuously issue more and more stringent standards concerning fluid waste disposed by, for example, industries into publicly owned treatment facilities. As a result, industries, which had heretofore only provided primary treatment to remove solid waste from fluid waste, are now faced with the necessity of providing secondary or biological treatment to remove soluble waste products from the fluid waste. Additionally, discharging industries are often faced with the burden of paying surcharges for discharging inadequately treated effluent into public works.
Biological waste treatment units have been very expensive to install and operate. Moreover, while numerous types of such treatment units are available, they share the common drawback of long waste treatment times and the resulting low volume/time yield of processed waste. In general, anaerobic microbial processes proceed very slowly and, moreover, due to the poor utilization of the carbon source resulting from the anaerobically growing cells (ATP-formation), the bacteria concentrations are low compared with those from an aerobic process. Consequently, continuous operation in, for example, a stirred vessel reactor, results in long residence times and low volume/time yields. By reducing the residence time to below the critical value, which corresponds to the reciprocal of the maximum growth-rate MY.sub.max, the microorganisms are washed out of the system and the process tends to become unstable if the reactor waste feed is not strictly regulated.
It is, however, possible to increase the volume/time yield if, on reducing the hydraulic residence time, the biomass is either retained in the reactor or is separated outside of the reactor and returned to the reactor vessel.
To be sure, processes based on the sedimentation of microorganisms have low efficiency and are strongly dependent on the settling ability of the cells, which can be impaired by the formation of biogas and by flotation.
While filtration is possible in principle, it requires, of course, special filtering units and is too expensive for treating significant quantities of liquid. An example of such a biological filtration treatment technique can be found in U.S. Pat. No. 3,732,160 which discloses the use of a submerged filter-horizontal flow method for wastewater treatment with a bacterial culture build up on a filter medium. The medium is graded progressively finer in the direction of the flow of the wastewater which is continuously recirculated through the medium.
Another example is European Patent Application No. 0,028,846 which teaches that an effective method of retaining biomasses within a reactor vessel is allowing the microorganism to grow on an inert carrier. In this technique, the waste liquid to be treated is fed continuously into a reactor charged with a suspension of bacteria, the process being such that a residence time is maintained which is below the wash-out point. Under these conditions, a process of selection takes place in which only those microorganisms, which develop suitably strong adhesion forces with respect to the support and which can take nourishment from the proffered carbon source, can multiply. The selectivity can also be a function of the presence of shear force effects, the intensity of which is a function of the waste liquid's rate of flow.
The above-described reaction in a fluidized bed reactor is advantageous in that a high incident flow velocity can be chosen to effect microorganism growth, in that no problem results from the separation of the biogas and in that approximately uniform conditions are present in the reactor. However, allowance must be made for the relatively high amount of abrasion product due to intense fluid bed motion and for the loss of bacteria as a result thereof. Moreover, a relatively large amount of energy is required to reach the fluidizing point.
On the other hand, the continuous flow-through fixed-bed reactor, as taught by the aforedescribed U.S. Patent, has the advantage of low abrasion loss and low energy consumption. However, only relatively low flow rates are possible and pockets of biogas form dead spaces in the reactor. There are the additional disadvantages of a blockage due to suspended solids, and also, a concentration and pH gradient within the reactor from the inlet end to the outlet ends thereof.
Several factors which are of importance in the decomposition of acetic acid in a waste solution have not been satisfactorily addressed by the techniques heretofore available in waste treatment. These factors include: the high chemical oxygen consumption of the acetic acid-containing waste product to be treated; its low pH, typically in the range of between about 2 to 4; the long doubling time of the microorganisms utilized in the process; a limited tendency to form large aggregates or flocculations; and the fact that the optimum pH for microorganism growth lies in a pH range of between about 6 to 7.2. The aforedescribed factors thus result in the necessity of establishing a long residence time for the waste liquid to be treated by the reactor bed material. Accordingly, the achievable volume/time yields are small. Additionally, it is necessary to monitor the pH of the waste liquid to be treated and typically bring it to the optimum value by the addition of a buffer or alkali before the liquid waste enters the reactor vessel. These limitations in the available waste treatment techniques render a process which can best be characterized as having unsatisfactory efficiency in secondary waste treatment.
It is, therefore, an object of this invention to improve the efficiency of an anaerobic decomposition process, particularly, an acetic acid-methanization process in a fixed-bed reactor.
It is also an object of this invention to provide a reactor apparatus for effecting the process of this invention.
It is yet another object of this invention to provide a process for the continuous anaerobical decomposition of organic compounds in an aqueous solution which does not require the addition of chemicals to maintain the pH of the reactor bed within the optimum range for the growth of bacteria.
It is still another object of this invention to provide a process and an apparatus by which a rapid rate of flow of liquid waste through a reactor bed can be established and maintained and whereby practically uniform pH conditions exist throughout the reactor bed.
It is a further object of this invention to provide a process and apparatus whereby anaerobic decomposition takes place throughout the entire reactor bed while substantially eliminating gas blockages within the reactor through satisfactory gas release above the reactor bed.